Field of the Invention
The invention relates to oil and fuel circuits in a turbine engine such as an airplane turbojet or turboprop.
Description of the Related Art
It is known to connect the oil and fuel circuits via heat exchangers in order to avoid the oil for lubricating members such as rolling bearings from overheating, the oil being cooled by exchanging heat with the flow of fuel that is fed to injectors mounted in a combustion chamber and to servo-valves for controlling variable geometry members such as variable pitch vanes.
To this end, a main oil/fuel heat exchanger is used that is arranged in the oil and fuel circuits downstream or upstream relative to an oil/air heat exchanger mounted in the oil circuit for cooling the oil leaving the core of the turbine engine. The oil/air heat exchanger has a stream of air passing therethrough or thereover that comes from outside the turbine engine.
A bypass pipe is mounted on this heat exchanger between the oil inlet and outlet, and a control valve serves to control the flow rate of oil passing through the oil/air heat exchanger and along the bypass pipe so as to control the temperature of the oil leaving the oil/air heat exchanger and entering the main oil/fuel heat exchanger.
Under cold operating conditions, such as during airplane takeoff, the valve is controlled to divert the flow of oil to the bypass pipe, which flow of oil then flows through the main oil/fuel heat exchanger. In this way, the oil does not pass through the oil/air heat exchanger so it is not cooled, thereby enabling the fuel to be heated and making it possible to guarantee a minimum temperature for the fuel that is fed to the injectors and the servo-valves.
Under hot operating conditions, e.g. at the end of takeoff or at the beginning of a cruising stage, the valve causes oil to flow through the oil/air heat exchanger, thereby serving to cool the oil and thus avoiding the oil coking. The cooled oil then flows through the main oil/fuel heat exchanger and delivers a smaller quantity of heat to the fuel, thus avoiding the fuel coking in the fuel injectors.
That type of configuration for the oil and fuel circuits is nevertheless not entirely satisfactory. Under cold operating conditions, the capacity of the main heat exchanger for transferring heat from the oil flow to the fuel flow can be insufficient. In a cold environment, water present in the fuel circuit can freeze, thereby disturbing the feeding of fuel to the injectors in the combustion chamber and greatly disturbing the operation of the servo-valves controlling variable geometry members.
Proposals have already been made to add a secondary oil/fuel heat exchanger upstream from the oil/air heat exchanger in the oil flow direction for the purpose of heating the fuel that is used as hydraulic fluid for controlling the variable geometry members. In such a configuration, the secondary oil/fuel heat exchanger is mounted immediately downstream from the oil recovery pump that recovers oil from lubricated equipment so that the heat exchanger has hot oil passing therethrough in order to provide a good transfer of heat to the fuel flow. Since the flow rate of fuel used as hydraulic fluid is less than the flow rate of fuel passing through the main oil/fuel heat exchanger, it is thus easy to heat it and to eliminate any risk of freezing under cold conditions.
Nevertheless, under hot operating conditions, the secondary oil/fuel heat exchanger contributes significant heating to the fuel fed to the servo-valves, thereby increasing the risk of the fuel overheating and thus of the fuel coking.
Introducing a secondary oil/fuel heat exchanger in series with the main oil/fuel heat exchanger and the oil/air heat exchanger gives rise to an increase in head losses in the oil circuit. As a result the oil recovery pumps arranged upstream from the secondary oil/fuel heat exchanger are subjected to greater back pressure, which means that they need to be of greater dimensions and thus of greater weight in order to conserve the same performance.
The oil and fuel circuit configurations described above are described in patent applications FR 2 951 228 and FR 1 061 138 in the name of the Applicant.